Detecting Archaeological Features with Airborne Laser Scanning in the Alpine Tundra of Sápmi, Northern Finland
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remote sensing Article Detecting Archaeological Features with Airborne Laser Scanning in the Alpine Tundra of Sápmi, Northern Finland Oula Seitsonen 1,2,* and Janne Ikäheimo 2 1 Cultural Heritage Studies, University of Helsinki, 00014 Helsinki, Finland 2 Archaeology, University of Oulu, 90570 Oulu, Finland; janne.ikaheimo@oulu.fi * Correspondence: oula.seitsonen@helsinki.fi Abstract: Open access airborne laser scanning (ALS) data have been available in Finland for over a decade and have been actively applied by the Finnish archaeologists in that time. The low resolution of this laser scanning 2008–2019 dataset (0.5 points/m2), however, has hindered its usability for archaeological prospection. In the summer of 2020, the situation changed markedly, when the Finnish National Land Survey started a new countrywide ALS survey with a higher resolution of 5 points/m2. In this paper we present the first results of applying this newly available ALS material for archaeological studies. Finnish LIDARK consortium has initiated the development of semi-automated approaches for visualizing, detecting, and analyzing archaeological features with this new dataset. Our first case studies are situated in the Alpine tundra environment of Sápmi in northern Finland, and the assessed archaeological features range from prehistoric sites to indigenous Sámi reindeer herding features and Second Word War-era German military structures. Already the initial analyses of the new ALS-5p data show their huge potential for locating, mapping, and assessing archaeological material. These results also suggest an imminent burst in the number of known archaeological sites, especially in the poorly accessible and little studied northern wilderness areas, when more data become available. Citation: Seitsonen, O.; Ikäheimo, J. Detecting Archaeological Features Keywords: archaeology; airborne laser scanning; LiDAR; Finland; Lapland; Sápmi; tundra with Airborne Laser Scanning in the Alpine Tundra of Sápmi, Northern Finland. Remote Sens. 2021, 13, 1599. https://doi.org/10.3390/rs13081599 1. Introduction Academic Editor: Dimitris Kaimaris Finland is the most forested of all the Nordic countries, with nearly 80 percent forest cover [1]. Owing to this, the use of aerial and satellite remote sensing, for example for Received: 19 March 2021 detecting crop marks, has been of limited use in the country, as less than 10 percent Accepted: 16 April 2021 of the whole country is covered by agricultural land. Thus, as soon as airborne laser Published: 20 April 2021 scanning (ALS/LiDAR, light detection and ranging) data became available in 2009 from the Finnish National Land Survey (NLS), they were also eagerly adopted by Finnish Publisher’s Note: MDPI stays neutral archaeologists. ALS data and methods in Finland originated from the needs of forestry with regard to jurisdictional claims in research and industry, e.g., [2], but the development of archaeological approaches was published maps and institutional affil- initiated immediately by the researchers from the University of Helsinki and the Finnish iations. Heritage Agency (FHA) [3–5]. Since 2009, the open access NLS airborne laser scanning 2008–2019 dataset (henceforth ALS-0.5p) [6] has gained a permanent place in the Finnish archaeologist toolbox and is routinely used, for instance, for planning surveys [1]. However, the low resolution of the Copyright: © 2021 by the authors. ALS-0.5p data, 0.5 points/m2, has dictated what kind of archaeological features can be Licensee MDPI, Basel, Switzerland. surveyed with it. It has proven useful for locating various kinds of pit and mound features This article is an open access article down to 2–5-m diameter, depending on the local conditions, such as Stone Age house pits, distributed under the terms and prehistoric pitfall traps, charcoal kilns, and modern conflict archaeological structures such conditions of the Creative Commons as trenches and dugouts [7–12]. However, owing to its low-resolution, the ground-truthing Attribution (CC BY) license (https:// of ALS observations has shown that numerous small-scale features had gone unnoticed, creativecommons.org/licenses/by/ and the material was most useful for directing fieldwork to new areas, e.g., [5,7]. However, 4.0/). Remote Sens. 2021, 13, 1599. https://doi.org/10.3390/rs13081599 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 1599 2 of 20 the open access ALS data have encouraged many archaeology enthusiasts to explore them actively as a hobby, and hundreds of new sites have been reported to heritage authorities as a result of this citizen–science engagement and co-operation [1,9,13,14]. In the summer of 2020 the situation changed strikingly, when the NLS launched a new countrywide ALS survey for a higher resolution elevation model with 5 points/m2. In this paper we present the first results of applying this newly available airborne laser scanning 5p (henceforth ALS-5p) material in archaeological studies. This research is part of a wider LIDARK consortium project coordinated by the FHA in association with the University of Oulu and funded by the Ministry of Agriculture and Forestry [15]. LIDARK project was launched in March 2021 and aims at developing semi-automated approaches for visualizing, detecting, and analyzing archaeological features with the ALS-5p dataset [10,16]. Because ALS-0.5p data have been openly available for over 10 years, no archaeological ALS missions were flown in Finland until 2019 when very high-resolution unmanned aerial vehicle (UAV) ALS surveys from drones were initiated [1]. Here we discuss the archaeological applications of the new ALS-5p dataset from the perspective of: (1) locating archaeological features based on both visual analyses and semi-automated detection using machine learning; (2) locational accuracy of the features; and (3) extraction of feature attribute information based on the DEMs derived from the ALS-5p point clouds. Point clouds from the first ALS-5p production areas were processed and quality checked by the NLS towards the end of 2020, and we received access to them early in 2021. By 2025, most of the country should be scanned with the higher resolution (northernmost parts by 2031) (Figure1). The presented case study is situated in northern Finland in the Alpine tundra landscape of Eanodat, in Sápmi, the homeland of Europe’s only indigenous people Sámi (thus Northern Sámi (SáN) placenames are used throughout) (Figure1). We discuss various types of archaeological localities, mostly with different-sized pit features with shallow embankments surrounding them, that range from Stone Age pitfall traps to historical-era indigenous Sámi reindeer herding sites and Second Word War-era German military structures. In the future, as new data become available from the NLS, the analyses will be continually expanded over large areas, eventually covering thousands of square kilometers, and the used methods will be continuously developed and refined accordingly. Figure 1. (A,B) Location of the Ropijärvenperä production area in Sápmi (Finnish Lapland), northernmost Europe (high- lighted with a red dot) (Background © Esri). (C) Finnish National Land Survey timetable for ALS-5p data collection, Ropijärvenperä with a red outline (EPSG:3879 ETRS89/GK25FIN). (Illustration: Oula Seitsonen). Remote Sens. 2021, 13, 1599 3 of 20 2. Archaeological and Remote Sensing Materials and Methods Our research material consists of the ALS-5p point clouds scanned in the summer of 2020 by the NLS in the about 1000 km2 Ropijärvenperä production area situated in Sápmi, northernmost Finland [17,18], and the archaeological data known from that area [14,19]. In this paper we discuss the results of the analyses of eight case study areas altogether covering 21 km2 (Figure2: 1–8, Table1). Below we describe first the archaeological material and the study area, the characteristics of the Ropijärvenperä ALS-5p dataset, and then the methods used for different analyses. Table 1. Case study areas 1–8 and the used ALS-5p tiles [17] and orthophotos [20]. Case Study Area Area (km2) ALS-5p Tiles Orthophotos 1 Eanodat Njamatjávri 1.5 W3332H2_6, W3341G1_4 W3332H, W3341G 2 Eanodat Bienne Biera Mohkki 2 W3334F4_3, W3334F4_6 W3334F 3 Eanodat Hirvasvuohppi 6 W3334H4_5-9, W4112B2_1 W3334H, W4112B 4 Eanodat Mohkkeguoika 3 W3333H4_1-3 W3333H 5 Eanodat Gálggobuolžžat 2 W3333E4_6, W3333E4_9 W3333E V34444B4_9, V34444D2_3, V3444B, V3444D, 6 Eanodat Ádjajohka 3 W3333A3_7, W3333C1_1 W3333A, W3333C 7 Eanodat Cáiceˇcuolbma 2 W3333E3_1-2 W3333E 8 Eanodat Cáicegorsa 1.5 V3444H2_3, W3333G1_1 V3444H, W3333G Total area (km2) 21 2.1. Archaeological Material and the Ropijärvenperä Production Area Case study areas 1 and 3–8 were selected based on the presence of previously ground- truthed archaeological features that provide reference material for the ALS-5p analyses, and the case study area 2 based on observation of previously unknown archaeological traces during a cursory visual inspection of the area. These initial analyses cover mere 2 percent of this vast and little studied wilderness region and will be expanded in the upcoming studies to cover the whole area and beyond (Figure1). Only one previous archaeological survey has been carried out in this mostly roadless landscape by the Finnish National Board of Forestry (NBF) [19]. The NBF and FHA have registered altogether 462 archaeological features from the Ropijärvenperä production area, mostly trapping pits used for hunting wild reindeer along their seasonal migration routes in the past (Table2). Nowadays the area is mostly used by Sámi pastoralists as seasonal herding grounds for their semi-domesticated reindeer. However, 75 years ago there was an intensive and unparalleled burst of human activity in this area that left a marked archaeological signature in the landscape. During the Second World War Finland had joined forces with Nazi Germany in the fight against the Soviet Union, and in 1944–1945 Germans built in this area a major defensive line consisting of Sturmbock-Stellung and Eisbär-Stellung, which stretched across the “western arm” of Finland from the Swedish border to Norway that was occupied by the Germans [12,14].